Displays fabricated using OLEDs (organic light emitting devices) provide a number of advantages over other flat panel technologies. They are bright, colourful, fast-switching, provide a wide viewing angle, and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) light emitting diodes (LEDs) may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160. Examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343. Examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole injection material such as a polythiophene derivative or a polyaniline derivative.
OLEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a thin film transistor (TFT), associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.
FIG. 1 shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 40 to 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are widely available. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
A substantially transparent hole injection layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole injection layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from H. C. Starck of Germany. In a typical polymer-based device the hole injection layer 106 may comprise around 200 nm of PEDOT. The light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case, banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved or enhanced through the use of cathode separators (not shown in FIG. 1).
The same basic structure may also be employed for small molecule devices.
Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress. Alternatively, the displays can be encapsulated prior to scribing and separating.
To illuminate the OLED, power is applied between the anode and cathode by, for example, battery 118 illustrated in FIG. 1. In the example shown in FIG. 1 light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective. Such devices are referred to as “bottom emitters”. Devices which emit through the cathode (“top emitters”) may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent and/or using a transparent cathode material such as ITO.
Referring now to FIG. 1b, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of FIG. 1 are indicated by like reference numerals. As shown, the hole injection layer 106 and the electroluminescent layer 108 are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and across-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal or glass can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
Considerable effort has been dedicated to the realization of a full-colour, all plastic screen. The major challenges to achieving this goal have been: (1) access to conjugated polymers emitting light of the three basic colours red, green and blue; and (2) the conjugated polymers must be easy to process and fabricate into full-colour display structures. Polymer light emitting devices (PLEDs) show great promise in meeting the first requirement, since manipulation of the emission colour can be achieved by changing the chemical structure of the conjugated polymers. However, while modulation of the chemical nature of conjugated polymers is often easy and inexpensive on the lab scale it can be an expensive and complicated process on the industrial scale. The second requirement of easy processability and build-up of full-colour matrix devices raises the question of how to micro-pattern fine multicolour pixels and how to achieve full-colour emission. Inkjet printing and hybrid inkjet printing technology have attracted much interest for the patterning of PLED devices (see, for example, Science 1998, 279, 1135; Wudl et al, Appl Phys. Lett. 1998, 73, 2561; and J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660).
In order to contribute to the development of a full-colour display, conjugated polymers exhibiting direct colour-tuning, good processability and the potential for inexpensive large-scale fabrication have been sought. Poly-2,7-fluorenes have been the subject of much research into blue-light emitting polymers (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629; J. S. Kim, R. H. Friend, and F. Cacialli, Appl. Phys. Lett. 1999, 74, 3084; WO-A-00/55927 and M. Bernius et al, Adv. Mater., 2000, 12, No. 23, 1737).
Active matrix organic light-emitting devices (AMOLEDs) are known in the art wherein electroluminescent pixels and a cathode are deposited onto a glass substrate comprising active matrix circuitry for controlling individual pixels and a transparent anode. Light in these devices is emitted towards the viewer through the anode and the glass substrate (so-called bottom-emission). Devices with transparent cathodes (so-called “top-emitting” devices) have been developed as a solution to this problem. A transparent cathode must possess the following properties: transparency; conductivity; and low workfunction for efficient electron injection into the LUMO of the electroluminescent layer of the device or, if present, an electron transporting layer.
An Example of a top-emitting device is shown in FIG. 2. The top-emitting device comprises a substrate 202 on which an insulating planarization layer 204 is disposed. A via hole is provided in the planarization layer 204 so an anode can be connected to its associated TFT (not shown). An anode 206 is disposed on the planarization layer 204 over which well-defining banks 208 are provided. The anode 206 is preferably reflective. Electroluminescent material 210 is disposed in the wells defined by the banks and a transparent cathode 212 is deposited over the wells and the banks to form a continuous layer.
However, there are very few conductive materials that are transparent at anything above very low thicknesses. One such material is indium tin oxide (ITO), and thus examples of transparent cathodes disclosed in the art include MgAg/ITO disclosed in Appl. Phys. Lett 68, 2606, 1996 and Ca/ITO disclosed in J. Appl. Phys. 87, 3080, 2000.
In these examples a first, thin layer of metal (or metal alloy in the case of MgAg) provides electron injection. However, the thinness of this layer is such that lateral conductivity is poor. A layer of ITO is necessary because it retains transparency at higher thicknesses, thus improving lateral conductivity of the cathode.
However, ITO is deposited by the high energy process of sputtering which is likely to cause damage to the layer(s) it is deposited onto. Given this, and the constraints in terms of alternatives to ITO, it would therefore be desirable if the need for a separate layer of transparent conductive material can be obviated.
Bus-bars are a well known method of increasing the conductivity of a conductive layer (see, for example, U.S. Pat. No. 6,664,730), providing a thickening of the metal away from the active region. However, unless these bus-bars are transparent, it will be immediately apparent that their use in top-emitting devices will reduce the emissive area of pixels in the same way that active matrix circuitry does for bottom-emitting AMOLEDs, thus reducing the advantages associated with said devices.
Inkjet printing of electroluminescent formulations is a cheap and effective method of forming patterned devices. As disclosed in EP-A-0880303, this entails use of photolithography to form wells that define pixels into which the electroluminescent material is deposited by inkjet printing. In WO 2006/123126, the present applicant has provided a solution to the problem of trying to enhance the conductivity of these thin transparent cathode layers in top-emitting devices without reducing emissive area of pixels by utilising the well-defining resist banks to provide structures on which a patterned metal layer can be deposited to give bus-bars.
A layer of metal on the top surface of the well-defining layer provides bus-bars that are able to enhance the conductivity of the transparent cathode layer which it is in contact with. Because the bus-bars provided by this metal layer are disposed on areas of the device that are already non-emissive due to the presence of the well-defining banks, the conductivity of the transparent cathode layer is enhanced without reducing the emissive area of the pixels.
In the method for manufacturing a top-emitting display described in WO 2006/123126, an anode layer is deposited on a substrate, well-defining bank material is deposited over the anode layer, and metal material for the bus-bars is deposited over the well-defining banks. The bank material and the metal material are patterned such that the metal forming the bus-bars is only located on the tops of the banks. Organic electroluminescent material can then be deposited in the wells over the anode material and a transparent cathode is then deposited over the electroluminescent material and the bus-bars on top of the banks.
It is an aim of the present invention to provide an alternative to the invention described in WO 2006/123126.